Conventional sensors of this type have an optical plate which has both convex lens structures and optical waveguides formed therein. The lenses focus the light incident on the pane, which is then guided through the optical waveguides and onto the photoreceivers. With its lens and optical waveguide structures that effect multiple deflections of the light beams, the optical plate has a fairly complex three-dimensional shape in adaptation to the spatial arrangement of the photoreceivers that are usually mounted on a printed circuit board. The complexity is especially great and a particularly large amount of space is required when the sensor comprises a plurality of sensor units, for example a rain sensor and light sensors.
A very much lower complexity and space requirement are achieved when an optical plate is used in which the lens structures are Fresnel lenses. A sensor of this type is the subject matter of unpublished document DE 10 2007 036 492. 1.
Optical sensors typically have a light directional characteristic which is dependent on the desired function. To the extent possible, rain sensors should only receive light which originates from a pulsating light source and is totally reflected on the windshield. Ambient light should be suppressed to the extent possible because it could only affect the measurement of the rain-related events. Light sensors, on the other hand, are designed to selectively receive light incident from specific directions and traversing the windshield, such as, e.g., frontally from a narrow solid angle or else non-directionally. The desired directional characteristic may be attained by taking various measures all of which, however, are involved and require additional space.